Add image upload support (/image API)

[tanysheng]

Documentation: https://serpapi.com/image-api

[Copilot]

Pull request overview

Adds first-class Image API support to the Java client, enabling multipart image uploads to /image and documenting how to use the returned image_id with engines like Google Lens.

Changes:

• Introduces SerpApi.uploadImage(...) overloads for Path and byte[], delegating to a new multipart HTTP helper.
• Adds SerpApiHttp.postMultipart(...) to send multipart/form-data requests containing text fields + binary image data.
• Adds offline tests for upload behavior and updates README/README.md.erb with Image API usage examples.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/test/java/serpapi/ImageApiTest.java	Adds offline tests validating upload parameter handling, multipart structure, and error propagation.
src/main/java/serpapi/SerpApiHttp.java	Adds multipart upload implementation used by the Image API client method.
src/main/java/serpapi/SerpApi.java	Adds uploadImage public API and refactors JSON parsing into a shared helper.
README.md.erb	Documents Image API usage in the README template.
README.md	Documents Image API usage in the generated README.

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